Fibre dewatering wakes up and smells the coffee

The physics of so many people’s indispensible morning gadget, the cafetiere or French Press coffee maker, could be a key tool for improving the efficiency of an increasingly important industrial process – removing water from natural fibres. An Anglo-Canadian team from the Universities of Cambridge and British Columbia has discovered how the microscopic structure of a suspension of natural fibres influences how it behaves when it is compressed, which they believe will help to optimise industrial processes involving dense suspensions, such as papermaking, waste treatment and an increasing number of processes based around making new materials from cellulose.

The behaviour of fibrous suspensions, where the volume of the suspension is much larger than the size of an individual fibre, is complex, as the fibres and the fluid in which they are suspended interact with each other. Particularly complex is the way that the fibres react to and transmit stress, explained Duncan Hewitt of the department of applied mathematics and theoretical physics at Cambridge. Using an analogy to a cafetiere was vital to the groups modelling, he added. “Using these devices, we were able to measure all of the empirical relationships required for the two-phase model," Hewitt said. "Then we tested the model under industrially relevant, but not widely studied, conditions of rapid compression using a French press geometry.” The team describes their work in the American Institute of Physics journal, Physics of Fluids.

The researchers found that a suspension of nylon fibres compressed in a single direction agrees with their model. As the suspension was compressed with a permeable piston, the concentration of fibres near the piston increased, which meant that the force needed to move the piston also grew. "But for cellulose fibres, the local solid fraction is more uniform,” Hewitt said. “The fibres spread more evenly -- rather than clogging up near the permeable piston, which our model predicts."

Such systems are getting more important, Hewitt said. While papermaking and removing water from sludges have encountered these issues for many years, recently manipulating suspensions of natural fibres has become a more common process. “There is a burgeoning economy, commonly termed the 'bio-economy,' based upon developing remarkable new products from cellulose or the constituents of plant biomass," Hewitt explained. "And bio-based materials made from cellulose, such as nano- or micro-crystalline cellulose, are at the forefront of replacing numerous oil-based plastics with natural ones." The cosmetics and food sectors also use suspenson dewatering processes, he added. "Our work helps with the design, scale-up, and operation of one of the most critically important units of operation within this process. It's also relevant for mining and wastewater treatment, for which the dewatering of suspensions and sludges is a key process." The next phase of the work will look at a different form of stress – shear, rather than compression.